Docs for installing the PHP agent

Please modify it to show a link to all relevant PHP agent downloads, especially "appdynamics-php-agent-x64-linux".

In another page of the sub-tree, there is a broken link to the wrong sub-domain.

I very much would have preferred a copy-pastable solution. Since the cookie is needed for downloading, just show a customized `curl` or `wget` command line that includes the cookie of the human reading the docs in the browser.

Please modify it to show a link to all relevant PHP agent downloads, especially "appdynamics-php-agent-x64-linux".

In another page of the sub-tree, there is a broken link to the wrong sub-domain.

I very much would have preferred a copy-pastable solution. Since the cookie is needed for downloading, just show a customized `curl` or `wget` command line that includes the cookie of the human reading the docs in the browser.